Evidence of Pressure-Induced Phase Transitions and Negative Linear Compressibility in Formamidinium Manganese-Hypophosphite Hybrid Perovskite

By combining two methods, Raman spectroscopy and high-pressure single-crystal X-ray diffraction, we demonstrate that the disordered, ambient pressure phase α of [NH2CHNH2]Mn(H2POO)3 is less compressible than the methylhydrazinium analog due to the formation of strong hydrogen bonds between the formamidinium cations and the framework. Above 4.0 GPa, the ordering of formamidinium cations and the collapse of perovskite cages lead to triclinic phase γ of space group P1̅. Phase γ exhibits a rare and important mechanical property, i.e., a negative linear compressibility of −7.82 ± 0.6 TPa–1 along the c axis. Raman spectroscopy indicates the presence of other phase transitions to phases δ and ε above 4.7 and 6.0 GPa, respectively. These transformations occur due to the cumulative distortions of the chemical bonds and structural components of the manganese-hypophosphite framework.